SIERRA COLINA VILLAGE - Lake Tahoe NV | Facts, FAQs, and Links

Yes. After application of baseline and post-project assumptions to the nhc Supplemental Water Quality Analysis (Appendix AA of the Sierra Colina Final EIS), nhc calculated that implementation of the proposed Project will reduce pollutant loads relative to the baseline condition for all pollutants of concern to lake clarity (i.e. suspended sediment, phosphorus, and nitrogen) by between 28% and 70% (See pages 11-13 of the nhc Supplemental Water Quality Analysis (Sierra Colina Final EIS (Appendix AA).

To create a conservative comparison of pre- and post-project estimated pollutant loads in its analysis, the water quality benefits provided by the proposed Project to restore existing pollutant sources on the parcel (i.e., restoring the SEZ utility road; decommissioning the dirt trails through the SEZ; and stopping runoff through the unauthorized discharge pipe and stopping further erosion of the parcel and Lake Village Drive from public runoff from Echo Drive dumping onto the parcel) are not included in nhc’s analysis. If all of the other water quality benefits which were excluded from the nhc water quality analysis (because nhc could not calculate them in a scientifically precise manner), were calculated and included in the nhc water quality analysis, the project would show even more substantial quantifiable water quality improvements.

In its analysis, nhc calculated the mitigated water pollutant loads caused by the TMDL Treatment Tier 2 Stormwater Management Plan for Sierra Colina and added that pollutant load to the water pollutant reductions resulting from the treatment on the Sierra Colina parcel at its expense of 26,000 square feet (0.6 acres) of Echo Drive, off-site public stormwater runoff. nhc modeled all pollutants of concern to lake clarity for both the baseline condition and post-project condition, including:

Table 4 below displays the results of the pollutant loading analysis for the baseline and the post-project conditions. The results indicate that implementation of the proposed Project will reduce pollutant loads relative to the baseline condition for all pollutants of concern to lake clarity (i.e. suspended sediment, phosphorus, and nitrogen) by between 28% and 70%. The post-project condition achieves a lower pollutant load relative to the baseline condition because:

1) the high level of water quality mitigation associated with the Sierra Colina TMDL Treatment Tier 2 Stormwater Management Plan, which minimizes the pollutant load increase attributed to the proposed development; and

2) the load reduction achieved by treating 0.6 acres of public storm water in an advanced storm water treatment system funded and maintained by Sierra Colina.

Parameter	Average Annual Estimate
Parameter	Baseline Condition (lbs)	Post-Project Condition (lbs)	Post-Project Percent Reduction in Loading
Fine Sediment Particle (FSP) Loading	116	56	52%
Total Suspended Solids (TSS) Loading	185	56	70%
Total Nitrogen (TN) Loading	4.3	2.4	43%
Dissolved Nitrogen (DN) Loading	0.8	0.6	28%
Total Phosphorus (TP) Loading	0.8	0.4	45%
Dissolved Phosphorus (DP) Loading	0.2	0.1	49%

Methods for numerical estimation of storm water pollutant loads in the Tahoe Basin are just emerging as components of the Lake Tahoe TMDL, and there is considerable uncertainty in these types of estimates. Of course, exact concentrations of pollutants and associated pollutant loads cannot be predicted with exact certainty because of the high degree of variability associated with a number of factors, which include but are not limited to runoff sources, BMP performance, hydrology, and water quality performance of maintenance activities. Nevertheless, nhc confidently concluded its Supplemental Water Quality Analysis as follows at pages 12 -13 (Sierra Colina Final EIS Appendix AA):

“Notwithstanding the uncertainty described, the SWMA is consistent with TMDL Treatment Tier 2 storm water controls proposed in the PRO Report. Among the treatment tiers evaluated in the PRO Report for urban areas, the Treatment Tier 2 storm controls were assigned the highest ranking of confidence for all performance estimates made because it applies a robust and comprehensive combination of source controls, advanced treatment, and frequent maintenance.

“Conservative assumptions were used to develop the comparison of baseline and post-project pollutant loads, and for this reason actual load reductions are expected to be higher than those used in the comparison. The comparison includes drainage from the Sierra Colina parcel and the portion of Echo Drive that will benefit from treatment in Project facilities. While the actual pollutant loads may vary from the modeled estimates provided in Table 4, the conservative basis of comparison provides a high level of confidence that the Project will result in a water quality improvement relative to existing pollutant loading from the Sierra Colina parcel and the portion of Echo Drive that will benefit from treatment in Project facilities.”